The planet Mars and Saturn’s moon Titan are the two most Earth-like worlds in the solar system. Each has an atmosphere, a surface that’s been carved by flowing liquids, and many other similarities to our home world.

One of those similarities is sand dunes.

The dunes on Mars are a good bit like those on Earth. They form as wind erodes rocks at the planet’s surface, breaking off tiny grains of material.

Sometimes, some of the sand in the dunes is swept up by planet-wide dust storms that are stirred by winds that can be as strong as a hurricane here on Earth. As the storms end, though, the sand settles back to the surface — some of it back atop the Martian dunes.

The dunes on Titan form in a different way. Titan’s surface is much colder than either Earth or Mars, and its atmosphere is denser. Clouds of methane and ethane float through the sky, raining droplets of hydrocarbons onto the surface. Some of these droplets can link together to form solid grains — the “sand” in Titan’s dunes.

Under Titan’s weaker gravity, the dunes can grow to enormous proportions — a mile wide, hundreds of miles long, and as tall as a 30-story building. In all, the dunes cover an area larger than the United States.

Mars and Saturn team up low in the western sky in early evening, with Mars to the left and Saturn to the right. Titan is visible through strong binoculars or a small telescope, and looks like a tiny star quite close to its parent world.

Script by Damond Benningfield, Copyright 2012

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